merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/t/lib-bash.sh

@@ -0,0 +1,17 @@
+# Shell library sourced instead of ./test-lib.sh by tests that need
+# to run under Bash; primarily intended for tests of the completion
+# script.
+
+if test -n "$BASH" && test -z "$POSIXLY_CORRECT"; then
+	# we are in full-on bash mode
+	true
+elif type bash >/dev/null 2>&1; then
+	# execute in full-on bash mode
+	unset POSIXLY_CORRECT
+	exec bash "$0" "$@"
+else
+	echo '1..0 #SKIP skipping bash completion tests; bash not available'
+	exit 0
+fi
+
+. ./test-lib.sh